1.Field of the Invention
The present invention relates to an air separation process and apparatus using cryogenic distillation. In particular, it relates to the production of nitrogen using a single column kept refrigerated by liquid injection (the sending of liquid nitrogen coming from an external source into the top of the column). The aim of the invention is more particularly to meet moderate and variable demands (typically 100 to 2000 Sm3/h) of high-purity nitrogen, that is to say nitrogen containing typically less than 0.1% oxygen. In the present specification, the flow rates in question are mass flow rates.
High-purity nitrogen is usually obtained cryogenically. For low consumptions, the construction of a conventional autonomous production unit represents a prohibitive level of investment in the case of automated installations, and a more limited level of investment but high labour costs in the opposite case. This always amounts to a high cost price of the nitrogen.
A more economical solution consists in using an evaporator, that is to say a liquid nitrogen tank of large capacity, for example several tens of thousands of liters, from which liquid nitrogen is withdrawn and vaporized. This solution is not very satisfactory from the energy standpoint, since the refrigeration energy contained in the liquid nitrogen is lost and, furthermore, it requires the presence relatively nearby of a liquid nitrogen production unit in order for the cost of replenishing the evaporator by a tanker lorry to remain moderate.
2. Related Art
Sometimes a gaseous nitrogen generator with liquid injection is installed with an emergency delivery system consisting of an evaporator, which makes it possible either to deliver gas to the customer if the apparatus is defective or to produce more gaseous nitrogen if the customer consumes more than the nominal production of the apparatus. The liquid from the emergency delivery system is generally vaporized in an atmospheric heater as may be seen in EP-A-0452177.
When there is a peak in consumption by the customer, liquid nitrogen from the storage tank is vaporized in an atmospheric exchanger (or a water pool) in order to top-up with nitrogen molecules, which nitrogen will be mixed with the nitrogen product output by the cryogenic apparatus. The refrigeration power of the liquid is therefore lost.
Liquid nitrogen also serves to keep the apparatus cold, by liquid injection. The amount of liquid nitrogen sent to the apparatus under steady operating conditions is about 3% of the nitrogen flow produced by the apparatus.